1. Field of the Invention
The present invention relates to semiconductor devices. More specifically, the present invention relates to fabricating Dual Conducting Floating Spacer Metal-Oxide-Semiconductor Field Effect Transistors (DCFS MOSFETs) using a process that is compatible with manufacturing conventional MOSFETs on the same wafer.
2. Discussion of the Related Art
Semiconductor non-volatile memory (NVM) and, particularly, electrically-erasable, programmable read-only memories (EEPROMs) are used in a wide range of electronic equipment (e.g., computers, telecommunications hardware and consumer appliances). In general, EEPROMs are often used to store firmware and data, as the content is maintained even with power off and may be altered as needed. The flash EEPROM may be regarded as a specially configured EEPROM that may be erased only on a global or sector-by-sector basis. The application of flash memory as a mass code and data storage device has placed additional demands on reduction of its cost-per-bit and increase in the required total memory density. In conventional NVMs, a one-to-one relationship exists between memory density and number of memory cells. The ability to store more that one bit in a single NVM cell is important for cost per bit reduction.
Single cell multi-bit NVM devices are known in the art. Most multi-bit NVM devices utilize multi-level thresholds to store more than one bit per memory cell, with each threshold level representing a different state. For example, a memory cell having four threshold levels can store two bits of information. Others physically store charge in separated regions of the NVM device. For examples, U.S. Pat. No. 5,021,999, issued to Kohda et al., teaches a non-volatile memory cell formed by an MOS transistor having a floating gate with two electrically separated segments. The memory cell can store three levels of data: neither segment is charged, charge injected into either one of the two segments, and charge injected into both segments.
As another example, U.S. Pat. No. 6,011,725 issued to Eitan teaches nitride read-only memory (NROM), which stores charge in nitride traps located respectively near the left and right regions of the memory cell. The left bit is read in the same manner as the right bit, except that the source and drain electrodes are reversed. Although the NROM-type NVM devices (MirrorBit™, available from Spansion) have been in production, the challenges of controlling nitride film properties and scaling down device geometry still remain. One challenge is to fill and to remove charge from inhomogeneous nitride traps in well-controlled areas above the device channel region. However, conventional programming methods, such as hot carrier injection (HCI), Fowler-Nordheim tunneling, and band-to-band tunneling cannot inject carriers into the nitride trap areas as precisely as desired because of programming non-uniformity and device inhomogeneity. Such non-uniformity can lead to a wider threshold voltage distribution and mutual threshold voltage interference between the source and drain terminals during programming. Furthermore, erasing charge from a charge-trap type NVM device requires removing electrons from the trap sites, or neutralizing them by injecting holes into the trap sites.
To remove stored electrons from the nitride traps using the Fowler-Nordheim tunneling mechanism requires a long erasing time and a relatively thinner, high voltage-limited nitride film, due to the adverse effect of reversed charge injection from the control gate. The charge transport mechanism in a nitride film, known as the Frenkel-Poole emission, is the thermal excitation of trapped charge from an inhomogeneous trap-potential medium. Thus, after erasure by applying a high electrical field to the nitride film, a wide and less controllable residual charge distribution remains. In reading, such a device is known to accumulate charge in its dielectric films, so that the sub-threshold slope of the MOSFET device is degraded, thereby resulting in a larger off-leakage current.
Precise injection of holes into the electron trapped sites is required to neutralize the trapped electrons. However, due to inhomogeneous depths of trap potentials, both electrons and holes may remain in nitride film trap sites even with the same injection point. The residual charge accumulation effect from programming and erase cycles may lead to a severe degradation in an NVM device. These issues of the NROM NVM device is expected to worsen, as the trapped electron area has to increase to achieve a higher threshold voltage shift and the device channel length has to be shortened to scale down the device.